The present invention relates generally to communication systems and, more particularly, to communication systems in which information is transmitted from one point to another on or near the earth's surface, by way of one or more intermediate nodes or stations. An intermediate node may be an earth-orbiting satellite, a high-altitude platform or, in the case of terrestrial system, an aggregation node where signals from multiple users are aggregated and forwarded. In this description, the invention will be described in the context of a satellite communication system, but it should be understood that the equivalent function of a satellite may be performed by a high-altitude platform or an aggregation node on the ground. This description also uses the terms “uplink” and “downlink” to refer to signal transmission to and from a satellite, respectively, but it should also be understood that use of these terms is not intended to limit the invention to a satellite communication system.
In the context of satellite communications, systems of the general type referred to above are sometimes referred to as “bent pipe” systems, in which an orbiting satellite functions essentially as a transponder, receiving data over multiple uplink channels and transmitting the information back to the ground over multiple downlink channels. Conventionally, the multiple uplink and downlink channels have separate radio-frequency (RF) carriers that are frequency division multiplexed (FDM) for uplink and downlink transmission. The multiple uplink carriers are usually transmitted from multiple uplink sources. For uplink reception on the satellite, the received composite uplink signals are separated by frequency filtering into multiple carriers, which are separately processed and routed. For downlink transmission from the satellite, the separate signals are recombined into a composite FDM signal and subsequently transmitted.
Although this FDM transmission technique has known advantages, the downlink aspect of FDM transmission suffers from a significant drawback. Amplifiers used in FDM transmitters must be “backed off” to run below the saturation point in their performance characteristic to ensure they operate in a linear range in which unwanted intermodulation products are minimized. These unwanted intermodulation products reduce the effective signal to noise ratio of the downlink signal. However, operation of amplifiers backed off from their peak power levels results in a power loss. Although the same considerations apply to the uplink, amplifiers in a ground station can be selected to provide a desired uplink power with minimal consideration of this power penalty. For downlink transmission, however, operation of amplifiers significantly below their peak power is a serious satellite efficiency and dc power concern.
Ideally, it would be desirable to operate downlink transmitter amplifiers at their peak power if the disadvantages that usually ensue from doing so could be avoided. The present invention is directed to this end.